(1) Field of the Invention
The present invention relates to a device for measuring displacements in a solid material, and more specifically to a device for applying uniaxial hydraulic pressure to the surface of an electro-active material while at the same time permitting a light source to be focused on the same surface in order to perform interferometric non-contact displacement measurements of the material under controlled conditions of pressure, temperature and applied voltage.
(2) Description of the Prior Art
The active elements of most sonar transducers consist of rings, disks or plates fabricated with electro-active (piezo-electric and electrostrictive) ceramics such as lead zirconium titanate (PZT) and with emerging materials such as the solid solution of lead magnesium niobate and lead titanate (PMN-PT). In a common configuration, these elements are bonded together with epoxy to form a stack that is then placed under a compressive load. When the stack is electrically driven, the applied compressive force opposes the tensile stress (internal strain) generated in the ceramic. This arrangement prevents the ceramic from going into tension and thus reduces the chance of failure due to fracturing.
Attempts to measure the electromechanical properties of stack elements often result in data that is difficult to interpret since the epoxy adhesive, metal electrodes and compression fixture tend to mask the properties of the ceramic. Therefore, a device for the direct characterization of the pre-stressed ceramic that eliminates the unwanted contributions from the stack assembly components is needed.
Currently, there exists a quasi-static apparatus used to determine the 33-mode properties of electro-active ceramics under simultaneous conditions of high electrical drive, electrical bias, compressive load and temperature.
With the above-mentioned quasi-static apparatus, a sample with dimensions of 2 mm×2 mm×10 mm. (an aspect ratio of 5:1 ensures 33-mode operation) is placed under a unidirectional compressive load along its length. The pre-stress is applied over a range of 0 to 10 ksi with a pneumatic piston designed to have low mechanical loss and low ac stiffness so that a “constant stress” boundary condition is met. The entire apparatus is placed onto a thermal control system in order to obtain data versus temperature. The sample is then electrically driven with a 10 Hz sine wave of the order of 2.0 Mv/m. The charge versus the applied field is measured using an integrating capacitor, and the longitudinal strain versus the field is measured with strain gauges attached to the sides of the sample. From these measurements, the large signal dielectric constant ∈33T the piezoelectric constant d33, and the coupling factor, k33, can be calculated as a function of drive signal, bias field, pre-stress and temperature. Young's modulus is obtained from the measurement of strain versus applied stress.
The device described above has several limitations. The required geometry and small sample size often cause problems with mechanical alignment, and under compressive load, samples are prone to mechanical cracking and electrical breakdown. Precise attachment of the strain gauges to the samples is difficult, affecting the reproducibility of the measurements from sample to sample. Furthermore, the gauges introduce stray capacitance, and due to their close proximity, exhibit electrical cross talk and promote electrical discharge arcing. Since temperature is controlled via an environmental chamber, long equilibration times are required before data can be acquired. In addition, temperature gradients within the chamber also affect the ability to repeat the measurements. For the most part, the prior art apparatus lacks the reliability and precision that is necessary to characterize electro-active ceramics in a reproducible and efficient manner. What is needed is a device capable of applying uniaxial hydraulic pressure to the surface of an electro-active material while at the same time performing non-contact displacement measurements of the material under controlled conditions of pressure, temperature and applied voltage.